This invention relates to an extractor designed to increase and regulate natural ventilation in industrial and/or agricultural buildings, such as industrial machining sheds, livestock-breeding farms, greenhouses and the like.
It is known that such buildings produce heat and harmful gases or fumes which must be continuously eliminated by means of a suitable air changing system. For obvious economic reasons, this problem is normally solved using ventilation systems with natural air circulation, which are simply called "static extractors". Only in particularly difficult cases, or in the presence of poison gases, use is made of ventilation systems with forced air circulation, equipped with appropriate ventilating fans. It should however be remembered that such fans--even apart from the costs--always cause localized air draughts which are harmful in some cases (for instance, in greenhouses), and for the health of the environment.
The aforespecified static extractors are usually installed at the top of a building (either on the summit of the roof or on the highest part of the side walls) for sucking stale air from the same. The intake of fresh air is instead guaranteed by a number of flap doors, or other similar openings, installed in the lower part of the building.
These extractors must simultaneously satisfy two fundamental and contrasting requirements, that is, on one hand they must prevent external atmospheric agents from propagating inside the building and, on the other hand, they must guarantee the outlet of the air stream which is extreme even and the flow of which varies according to requirements.
For what concerns the first problem, while it is relatively simple to prevent the inlet of water and snow, it is instead very difficult to prevent the access of wind, unless greatly reducing the free outlet sections of the extractor, thereby reducing the flow of outgoing air. On the other hand, it is particularly important to prevent the incoming wind from causing a reversal of the flow of outgoing air, and a consequent inlet of cold air from the top of the building, rather than from the openings provided for this purpose in the lower part of the building itself. In fact--particularly in the winter season, when the flow of outgoing air is reduced to a minimum (as will be better explained hereinafter)--the cold air eventually coming in from the top draws along with it the masses of stale rising air, recycling them with no possibility of control and hampering the regular change thereof. Furthermore, it causes undesired air draughts and an uncontrollable increase of the amount of heat required for keeping the environment in the desired conditions of temperature.
For what concerns the second problem, namely the control of the flow of fresh air coming out of the extractor, it should be noted first of all that it must be possible to vary said flow between a minimum value, typical of the winter season, and a maximum value which is instead typical of the summer season.
The minimum winter value is determined by the requirement to remove the least possible amount of heat in order to maintain a high difference between the indoor and outdoor temperature, while guaranteeing the dilution of the polluting gases circulating in the air and, at the same time, ensuring healthy environmental conditions. The maximum summer value is instead determined by the need to remove as much as possible of the heat produced within the building, in order to ensure the smallest difference between indoor and outdoor temperature (for instance, 4.degree. or 5.degree. C.).
It should at once be noted that the ratio between maximum and minimum flow is quite considerable and it can reach values of fifty or more.
Different types of static extractors have been proposed up to date, which have given preference to the solution of either of the aforementioned problems, while only one extractor has been proposed, apt to successfully overcome both problems, but the spreading of which has been impeded by the exceedingly high cost thereof. We shall now briefly examine the main static extractors of known type.
Perhaps the most widely known and spread static extractor now on the market is the "Robertson" extractor. This is characterized by a metal device generally mounted on the summit of the roof, having an elliptical cross section, open on top, and being provided inside with an overturned V-shaped bent tile surface apt to prevent the inlet of inclement atmosheric agents and to discharge them sideways. This extractor, though being fairly economic, presents however a number of drawbacks, such as the following:
having fairly large free outflow surfaces for the stale air, it is extremely easy for cold air to enter from the leeward side and, furthermore, under particularly severe conditions, even with protection against weather inclemencies is unsatisfactory; PA1 in the event of abundant snowfalls, when it is out-of-work, it easily tends to clog with snow, with consequent overloads on the building; PA1 the extraction effect is acceptable when the wind blows perpendicularly to the axis of the extractor, whereas, when the wind blows along said axis, its efficiency drops to very low levels; PA1 its dimensions are very large, both in width and in height, as compared to the width of the roof opening it fits onto, whereby the weight of the extractor and the mechanical action thereon of the wind are such as to require--in most applications--that the roofing structure be specially reinforced, which notably reduces the economical advantages of this device. PA1 they can be mounted only on the roof of the building to be ventilated, and not on the side walls thereof; PA1 they are not apt to prevent the inlet of winds blowing crosswise, unless by closing the openings for the passage of the flow of outgoing air, thereby annulling the extracting action; PA1 they are so formed as to pile up snow inside in case of abundant snowfalls. This drawback is particularly felt in the northern regions and it occurs when the plants are out-of-work, failing the heat produced inside the building.
The original "Robertson" extractor has been the object of several modifications and improvements, which have given rise to a number of improved extractors, among which we can recall the extractors object of the U.S. Pat. Nos. 3,107,598 and 3,182,580 and of the German Pat. No. 2,156,189. In these extractors, an attempt has been made to eliminate the drawbacks present in the "Robertson" extractor, particularly for what concerns the inlet of wind in the extractor itself. This result has been achieved, both by dividing the free outflow section into several sub-sections and by providing closing means which should operate in the winter season. The partial results obtained with these extractors are however not compensated by the weight and bulk, still very high, as well as by the further complexity deriving from the provision of the new devices. Moreover, the operation of the closing means in reply to unforeseeable events, such as are determined by atmospheric agents (particularly wind), can only be done manually, with all the inconveniences deriving therefrom, both from the economical point of view and from the point of view of the results obtained.
The British Pat. No. 678,032 describes an extractor which has excellent characteristics for what concerns the impenetrability to atmospheric agents, particularly wind. This is obtained, however, thanks to a particularly tortuous and narrow path of the flow of outgoing air, whereby this apparatus is fit only for applications which require a scare flow of extraction air, or it has to be used in combination with motor driven ventilators.
Another type of extractor is described in the Swiss Pat. No. 371,879. In this case, the device has a fairly light and simple structure and its free sections are wide enough to allow even high outflow rates of the stale air. Nonetheless, the devices provided therein for preventing the inlet of rain are scarcely efficient and, furthermore, the access to winds blowing crosswise is prevented only closing all the openings, thereby renouncing to the extracting action.
It can finally be said that the heretofore considered extractors, though varying in their achievements which involve different structural problems and hence different costs, still have in common the following drawbacks:
As already mentioned, there is also a type of static extractor which has successfully solved the aforespecified problems. This extractor is commonly known as the "Mueller" or "Modified Mueller" extractor and it consists of a double layer of suitably shaped and spaced blades, as described in the Italian Pat. No. 883,144.
This last type of extractor has the characteristic of preventing the inlet of wind, no matter what direction it blows from, while it takes advantage of the depression caused by the wind--to a further extent in the leeward side of the building--in order to accomplish a particularly efficient extraction.
However, since this extractor (as, obviously, also all the previous ones) must clearly be dimensioned according to the maximum summer flow (the flow reduction in the winter season is in fact obtained by gradually closing the flap doors provided in the lower part of the building), and it is furthermore provided with a relatively low specific outflow surface, of relatively high unitary cost, it involves installation costs which are too heavy to allow its proper spreading on the market.
The object of the present invention is to overcome the aforespecified drawbacks by providing a device which--with a structure involving very limited costs and having characteristics of size and weight such as to present no problems of stability during installation--allows to adjustably increase the air flow during the summer season, as related to the minimal flow required in the winter season, while preventing the inlet of cold air in any weather conditions.